The re-engining idea has been around since the Boeing 757/C-17 engines went into service in 1982. The asymmetric issue can be dealt with. The last look was by the Defense Science Board in 04, I think, and chided the AF for using an economic model based on $1/USG fuel - not the $17/USG that it costs out of the back end of a KC-135.
It makes technical and economic sense but never happened and never will. The fighter generals simply never imagined that the B-52 could be this important for so long.